1. Field of the Invention
The present invention relates to ultra-high and extremely high vacuum measuring devices, and especially to a cold cathode device and a vacuum gauge having the cold cathode device used for measuring pressure in ultra-high or extremely high vacuum conditions.
2. Prior Art
Nowadays, high vacuum conditions are employed in many fields of endeavor, such as in simulation technology in aerospace, superconductor technology, nuclear fusion technology, ultra-low temperature technology, and huge particle accelerator technology. Vacuum gauges for measuring pressure in ultra-high and extremely high vacuum conditions are needed.
A BA ionization gauge invented by Bayard and Alpert in 1950 is widely used for measuring pressures in ultra-high vacuum conditions. Referring to FIG. 4, the BA ionization gauge includes a glass shell 4, a collector 2, a grid 5, at least one thermionic filament 1, and several filament leads 3 for connecting the filament 1. However, the gauge has a large volume and many electrodes. Many gas molecules and ions are adsorbed onto surfaces of the electrodes. When the vacuum pressure rises, the gas molecules and ions adsorbed on the surfaces of the electrodes can be released into the gauge. Therefore the BA ionization gauge can only be used in a dynamic vacuum system, and is not suitable for measuring pressure in ultra-high and extremely high vacuum conditions. A vacuum gauge marketed under the brand name AxTRAN by ULVAC technologies, Inc. is used for measuring pressure in ultra-high and extremely high vacuum conditions. The AxTRAN vacuum gauge can measure pressures of as little as 10—11 Pa. However, the AxTR.AN vacuum gauge has a complex structure, and it is difficult to eliminate gases. Therefore the AxTRAN vacuum gauge has limited appeal in the marketplace.
In general, a gauge for measuring pressure in ultra-high and extremely high vacuum conditions should be small, and should have little release of gas. Further, a gauge used in aerospace should also be energy efficient. In particular, the gauge should be light in weight, and a corresponding power supply should also be light in weight.
In the 1980s, Chen, Pi-Jin and Li, You-Ze, of the Department of Electronic Engineering of Tsinghua University, China, developed a saddle field gauge to measure pressure in extremely high vacuum conditions, based on a micro-ionization gauge. Refer to Science of Vacuum Technology, 1987, National Defence Publishing Company. The saddle field gauge can obtain a very long electron track by using a static saddle field to restrict electron vibration. This improves the sensitivity of the saddle field gauge.
In the year 2000, China Pat. No. 94100653.0 to Chen, Pi-Jin and Qi, Sing showed a micro-ionization gauge having a wide range, the gauge employing a field emission source as an electron emitter. The micro-ionization gauge includes a glass shell, an anode, an ion collector, a field emission source, and a metal film electrode. The field emission source employs a Spindt type field emission source instead of a thermionic cathode, for avoiding adsorbing and releasing of gases caused by chemical reactions on surfaces of a thermionic cathode. However, the Spindt type field emission source has a grid to control the projecting of the electrons. Therefore the sensitivity of the micro-ionization gauge is influenced by an electric field produced by the grid.
China Pat. No. 99109355.0 discloses an extremely high vacuum ionization gauge which has a very low rate of adsorbing and releasing of gases. The ionization gauge uses a field emission cold cathode, instead of various kinds of thermionic cathodes, to improve the sensitivity of ionization gauge. However, the cold cathode has a grid. The potential of the grid disturbs the entire electric field distribution of the ionization gauge, and distorts the symmetry of a saddle field in the ionization gauge. Therefore the vibration of electrons in the ionization gauge is asymmetrical, which reduces the sensitivity of the ionization gauge.
It is desired to provide a cold cathode device and a vacuum gauge having the cold cathode device which overcome the above-described shortcomings.